1. Field of the Invention
The invention concerns an apparatus for the visual matching or alternatively for the contactless measurement of samples, wherein effects dependent on the direction of observation are to be detected as well.
2. Description of Related Art
Apparatuses for color measurement and for color matching are known, in ordinary color measurement starting from a sample which is fixed relative to the measuring device and which is illuminated from various predefined angles and usually observed perpendicularly to the surface. In these apparatuses, the measuring devices are arranged in the immediate vicinity of the sample surface and only very small surface portions are viewed. For the color matching of smaller samples, small lighting booths are usual, e.g. table-mounted booths, in which is provided a stationary lighting system for illumination of the samples. In order to detect the different color effects, the sample surfaces, usually a sample to be measured and a reference sample, are presented to an observer from different angles and so also illuminated from different angles. This method has the drawback of poor reproducibility, because both the lighting angles and the angles of observation change in different observation situations, in particular color effects are difficult to detect, and moreover only very small samples can be matched.
From U.S. Pat. No. 5,604,586 is known an apparatus for color matching which comprises a concavely shaped plate for receiving a sample to be matched. Furthermore there is provided a sample holder for a reference sample, which is also concavely curved and which is pivotable about a common evolute of the surface of the plate and the surface of the sample holder. Above the samples is arranged a surface lighting apparatus whose radiating surface is convex and which irradiates the samples with uniform and diffuse light of predetermined intensity.
It is the object of the invention to find an apparatus for measurement or visual matching for small and medium-sized flat or slightly curved samples, which allows reproducible and clearly defined, different observation conditions with sufficiently similar lighting conditions, and which under the same conditions is suitable both for visual matching and for color measurements.
This object is achieved according to the invention by the characteristics of the main claim.
The apparatus according to the invention starts from a fixed observer location. From this location a surface element on the sample or the assembly of several samples is viewed at their edges adjoining each other, preferably in a basic position from the direction of the normal of the surface element. By the position of the sample is fixed a plane in which the sample is located. A change in the angle of observation is caused according to the invention by the fact that the sample is displaced in a linear direction until the desired angle of observation is reached as the angle between the direction of observation and the normal on the sample or surface element. According to the usual stipulations in many color matching regulations, the sample is illuminated in its basic position in the usual manner obliquely, preferably at 45xc2x0. This can be done by an almost parallel or narrowly focused beam path, e.g. with spotlights and parabolic reflectors. Particularly advantageous, however, and with a high correlation with the lighting conditions outdoors, is a beam path which is narrowly focused only in a main plane. Light distribution of this kind can be produced by linear light sources.
According to the invention the whole surface in which the sample or portions of the sample can be located is now illuminated at each location from the same oblique direction. This means that the lighting conditions on the sample remain constant even in case of displacement, with the result that reproducible matching dependent on the angle of observation is guaranteed.
When using linear light sources, the main plane of radiation in which maximum focusing is achieved is the plane which contains the normal on the sample and the straight line of displacement of the sample. The longitudinal axes of the lamps or light sources run perpendicularly to this main plane. If the sample is arranged vertically and displaced horizontally, the result is therefore vertically oriented lamps parallel to the sample. If the sample is likewise arranged vertically but displaced vertically, the result is horizontally oriented lamps parallel to the sample. These can be mounted for example on the ceiling or on side walls.
The lamps, if necessary with appropriate filtering, are selected in particular for visual matching such that their spectral distribution meets preset conditions, e.g. a color temperature of 6500 K and high enough index of metamerism in daylight. This is provided for example by certain fluorescent tubes.
With the same geometry, according to the invention the observer""s eye can be replaced by a sensor optical system, whose optical system spectrally assesses the beam density of the sample.